The invention concerns a mixing device for the mixing of, in particular, fluids and/or gas-phase measuring samples in a nuclear magnetic resonance (NMR) measuring apparatus with a high field magnet coil for the production of a homogeneous static magnetic field and a measurement volume within the measurement sample, whereby the measuring apparatus includes a rod which is arranged coaxially to the main field magnet coil, is movable in an axial direction, exhibits, on one end, a piston projecting into the liquid and/or gas-phase measuring sample, and with the assistance of a drive coil arranged at the end facing away from the measuring sample, is moved in the axial direction by the flow of current through the drive coil with a spring element being provided for to move the rod into a rest position when no current flows through the drive coil.
A mixing apparatus of this kind is, for example, known from the article "A High-Pressure Probe for NMR Studies of Homogeneous Catalysis" by Velde and Jonas in "Journal of Magnetic Resonance", No. 71, pp. 480-484, 1987.
The high-pressure NMR measuring apparatus described therein serves for the investigation, with the assistance of nuclear magnetic resonance at high pressures, of chemical reactions which take place in a saturated gas-fluid mixture. From the theory of partial pressures it is known that, the higher the applied external pressure, the larger the fraction of gas which can be dissolved in the liquid. By means of the resulting chemical reactions, the saturated state of the mixture is destroyed and must be constantly adjusted.
The saturated state is an equilibrium state which depends on the temperature as well as the pressure. For this reason, for this type of measurement, it is necessary to have a variable constant-pressure apparatus in the range from 1 to 1000 bar with temperature regulation (typically in the region of -20.degree. C. to +180.degree. C.).
In order to quantitatively record the observed chemical reactions, it is necessary for the reactions to take place evenly and simultaneously within the entire liquid. It is therefore necessary for the gas and the fluid to mix with each other as quickly as possible and as evenly as possible and, in fact, before the reaction sets-in. An active mixing process is therefore essential for this type of investigation. Without an active mixing procedure, the mixing process would only take place by means of diffusion and would therefore transpire very slowly. It takes on the order of several days to achieve a saturated state through diffusion only. On the other hand, an active mixing procedure shortens the necessary mixing time by many orders of magnitude; the saturated state is achieved in only 10 to 60 seconds.
The mixing method should preferentially take place within the main field magnet of the NMR measuring apparatus in order to avoid additional losses in time. In this fashion the device is available for further measurement as quickly as possible and is therefore capable of exactly recording the time development of the chemical reaction. During the NMR measurement procedure, which takes place at constant pressure, it is necessary for the mixing procedure to be repeated from time to time. This is necessary since gas is used-up by the chemical reaction, and therefore new gas must be added and mixed-in in order to restore the required saturated state. Only a measuring procedure carried out in this fashion leads to reliable, reproduceable measurement values of the time development of the reaction.
This type of reaction time development measurement can be carried out at different temperatures and pressures which, in each case, are kept constant during the measurement procedure.
This mixing apparatus itself should, preferentially, be non-magnetic so that the necessary extremely high homogeneity of the magnetic field produced by the main field coil for the NMR measurement is not deteriorated due to the overlap of magnetic fields from the measuring apparatus. It is therefore essential that ferromagnetic or permanent magnetic materials be, by all means, avoided in the measuring apparatus or, when absolutely necessary, be arranged as far away as possible from the magnetic center of the NMR measuring apparatus.
In a known mixing method, the measuring sample to be shaken is initially removed from the main field magnet and subsequently is brought in again to the main field magnet in order to carry out the NMR measurement. A disadvantage of the known method is that the measuring sample begins to react following the shaking and before the NMR measurement has begun. In addition, the temperature of the measurement sample changes during this process and must, subsequently, be restored again to the desired constant value.
In another known measurement method, electric motors with permanent magnets are utilized in order to produce a mechanical shaking motion. A method of this type as, for example, described in the above cited article in JMR 71, 480-484 (1987) exhibits, however, a series of serious disadvantages:
The ferromagnetic and permanent magnetic materials contained in the electric motor distort the homogeneous field of the NMR main field magnet. In addition, the permanent magnet portion produces additional inhomogeneous fields.
Magnetic fields between the electric motor and the superconducting magnet system can lead to mechanical stability problems in the event that the two are too close to each other.
In order to keep the mutual influences between the electric motor and the NMR main field magnet as small as possible it is necessary that the electric motor be positioned very far away from the magnetic center of the NMR main field magnet. This necessarily leads to a large and cumbersome mixing apparatus.
In the event that the electric motor works outside of the main high pressure area in which the NMR measuring sample is located, mechanically movable high pressure feed-throughs are necessary in order to transmit the shaking motion. The latter are extremely difficult to produce, unreliable, and nearly impossible to realize at high pressure. On the other hand, an electric motor arranged together with the measuring sample in the high pressure region likewise leads to serious technical problems.
In contrast thereto it is the purpose of the present invention to present a mixing device of the above mentioned kind with which the homogeneous magnetic field of the NMR main field magnet is not encroached upon during the course of the NMR measurement having no mechanical forces present between the measuring device and the NMR main field magnet and which is capable of arrangement in spatial proximity to the NMR measuring magnet to work in a particularly reliable fashion in a high pressure region, wherein the entire apparatus is capable of particularly compact configuration.